Modern wireless communication devices typically employ a frequency synthesizer using phase locked loop technology to control the receive frequency of the device. A voltage controlled oscillator (VCO) provides a synthesizer output signal which is locked in frequency and phase to some multiple of a reference frequency provided by a reference oscillator. The synthesizer output signal is applied to a quadrature divider that processes the signal into an in-phase signal and a quadrature signal, the quadrature signal being 90 degrees out of phase with respect to the in-phase signal.
The in-phase signal is applied as a local oscillator signal, ILO, to a mixer in the in-phase or I channel of the communication device. The quadrature signal is applied as a local oscillator signal, QLO, to another mixer in the quadrature or Q channel of the communication device. These mixers may be called the I channel mixer and the Q channel mixer, respectively. The receive signal from an antenna is supplied to both the I channel and Q channel mixers. Thus, the I channel and Q channel mixers mix the receive signal with a local oscillator signal down to some intermediate frequency (IF). Other circuits couple to the I and Q channels to further process IF signals into baseband signals. Finally, the baseband signals are processed to provide an audio signal, data signal or other output signal that is supplied to an output of the communication device.
Unfortunately, image signals can be an undesired byproduct of the mixing process. When a mixer mixes a receive signal with a local oscillator signal, it is possible that two signals can be received, namely the desired signal and an image signal. These two signals lie in a frequency band separated by a frequency which is twice the intermediate frequency (IF) of the receiver. The receiver designer seeks to allow the desired or wanted signal to pass through the receiver unimpaired while completely rejecting the image signal. In one example wherein the local oscillator signal is at 1 GHz and the desired receive signal is at 1.1 GHz, it is possible that the receiver might receive not only the desired 1.1 GHz signal, but also a 0.9 GHz signal, namely the image signal. Image rejection can be a challenging problem in communication device design.
What is needed is a wireless communication method and device that provides improved image rejection.